1. Field of the Invention:
The invention is directed to optical communication systems in general and, more particularly, to bidirectional optical networks that include two or more independent bidirectional wavelength division multiplexed optical communication systems transporting wavelength division multiplexed optical signals in opposite directions over the same bidirectional waveguiding medium and which include an optical bridge for directing selected optical channels between the independent bidirectional WDM optical systems.
2. Description of the Related Art
As the need for communication signal bandwidth increases, wavelength division multiplexing (WDM) has progressively gained popularity for multiplying the transmission capacity of a single optical fiber. A review of optical networks, including WYDM networks, can be found in Ramaswamni et al., Optical Networks: A Practical Perspective (Morgan Kaufinan, .COPYRGT.1998), the disosure of which is incorporated herein by reference. Typically, wavelength division multiplexed optical communication systems have been designed and deployed in the long-haul, interexchange carrier realm. In these long-haul optical systems, a wavelength division multipexed optical communication signal comprising plural optical channels at different wavelengths travels in a singe direction on a single fiber (unidirectional transmission). Because the communication traffic in such systems commonly travels many hundreds of kilometers, the need for add-drop multiplexing of individual channels is infrequent (if at all), occurring at widely-spaced add-drop nodes.
Although the optical infrastructure of long-haul WDM optical systems can accommodate future traffic needs created by increased demand from traditional and multimedia Internet services, this traffic must first be collected and distributed by local networks. Currently, such local networks are structured to carry a single wavelength, time-division multiplexed (TDM) optical signal along a fiber network organized into various ring structures. To route the various components of the TDM signal, numerous electronic add-drop multiplexers are positioned along the fiber network. At each add-drop location, the entire optical signal is converted into an electrical signal; the portions of the electrical signal which are destined for that add-drop point are routed accordingly. The remaining portions of the electrical signal are converted back to a new TDM optical signal and are output through the electronic add-drop multiplexer. Thus, before a user can access the bandwidth-rich WDM long-haul transport networks, he must first pass through the bottleneck of the local networks.
Although unidirectional WDM optical systems are suitable for conventional long-haul interexchange carrier markets, metropolitan (local) communications systems require extensive routing and switching of traffic among various nodes positioned within interconnecting optical fiber rings. Consequently, smaller metropolitan markets require considerably more extensive add-drop multiplexing in order to successfully implement wavelength division multiplexing in their short-range systems. Further, in order to maximize the effectiveness of wavelength division multiplexing in these local areas, it would be useful to implement bidirectional WDM optical systems, e.g., to enhance network design flexibility. In a bidirectional WDM system counter-propagating WDM optical signals, each of which carry a number of optical channels, are carried on the same waveguiding medium, such as a single optical fiber. Implementation of a bidirectional system requires several considerations not present in the conventional unidirectional optical systems. Add-drop multiplexing in a bidirectional optical environment becomes considerably more complex since optical channels must be selected from each of the counter-propagating WDM optical signals. In addition to the difficulties posed by add-drop multiplexing channels from two counter-propagating WDM optical signals, there must also be techniques for directing channels from one independent bidirectional WDM optical system to another. For example, in a local metropolitan network, it would be desirable to optically transfer traffic among adjacent bidirectional rings.
Several bidirectional multiplexing designs have been proposed; however, none of these include techniques for optically routing traffic between independent bidirectional WDM optical systems. In U.S. Pat. No. 5,909,295, optical circulators are used to separate the counter-propagating optical signals that are further filtered down to individual channel wavelengths; this design appears to be predominantly directed to an end node in a WDM optical system. In many embodiments, expensive four-port (or higher) optical circulators must be used. Although optical channels are separated, there is no teaching or suggestion of signal recombination such that a bidirectional optical signal continues to propagate along a bidirectional transmission waveguide.
U.S. Pat. No. 6,130,765 a bidirectional add-drop multiplexer is described. The apparatus comprises two three-port main circulators inserted into a line fiber; the third port of each circulator is connected by auxiliary optical fibers to auxiliary circulators. While this patent depicts channel routing in a single bidirectional line fiber, it does not teach or suggest an optical device capable of routing optical channel traffic between two independent bidirectional line fibers belonging to independent bidirectional WDM optical systems.
Thus, there is a need in the art for a bidirectional device to permit optical channel routing between independent bidirectional optical communication systems. Such devices would permit effective implementation of bidirectional wavelength division multiplexing in local, metropolitan markets requiring high volumes of signal re-routing and allow creation of flexible network topologies.